High speed data encoded by orthogonal frequency division multiplexing (OFDM) will transfer into low speed data and be modulated. The modulated data is transmitted to sub-carriers where each of the sub-barriers is orthogonal to each other.
Inter channel interference (ICI) occurs due to decline of the orthogonality among the sub-carriers when there is an offset between a transmitting end and a receiving end. Thus, amplitudes and phases of transmitted signals become different.
As a result, a wireless local area network (WLAN) system has to estimate frequency offsets within preambles that includes short training field (STF) and long training field (LTF), and then compensates for the frequency offset according to the estimation.
The method for precisely estimating the frequency offset performed by WLAN system disclosed in Korean published patent number 10-2005-0034116 (published on Apr. 14, 2005) and comprises following steps: first, the WLAN system estimates and compensates a coarse frequency offset according to the length of the STF at the transmitting end. Second, estimating a fine frequency offset while applying the estimated coarse frequency offset to compensate the STF. Last, changing the compensating point according to the estimated fine frequency offset. Therefore, delay caused from calculation and compensation of the frequency offset is reduced.
However, the present technology as disclosed by the Korean published patent number 10-2005-0034116 cannot verify the estimation of the frequency offset because only training signals within one specific interval are utilized. Therefore, the estimations are not precise enough.